1. Field of the Invention
This invention relates to a resin serving as a material of a polyethylene container which is suitably used in the industrial fields for the production of semiconductor units and fine machining parts, or in the production of high purity chemicals, and medical/pharmaceutical products which even when filled with a high purity chemical, permits substantially no transference of fine particles to the chemical or substantially no dissolution of metals into the chemical.
2. Description of the Related Art
Recently, with the remarkable technical progress in the field of electronic industry, the demand for high purity chemicals becomes acute. Such high purity chemicals are essential for the fabrication of electronic circuits such as large-scale integrated circuits or LSI and others. More specifically, chemicals used for the processes such as cleaning/etching of wafers, wiring/etching of insulating films, cleaning of tools, and preparation of developing solutions, resist diluents, resist removing solutions and drying solutions include sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid, ammonium fluoride, an aqueous solution of hydrogen peroxide, isopropyl alcohol, xylene, Tetramethylammonium hydoxide, methanol, acetic acid, phosphoric acid, aqueous solution of ammonia, Propylene glycol monomethyl-ether acetate, Dimethyl sulfaxide, N-methyl-2-pyrrolidone, ECA, ether lactate, etc. Conventionally, the substance used as a material of the container for those high purity chemicals include polyethylene resins because they are sufficiently resistive to chemicals and impact, and inexpensive. However, with conventional containers made of polyethylene resins, the resin may dissolve into the chemical within, or contaminants including deteriorated resin may pollute the content, which has posed a problem. Thus, the previous resins have a limitation as a material of the container for high purity chemicals. Indeed, with the further compaction of ultra-fine LSIs, the current demand for the tolerable limit of metal impurities falls to 0.1 PPB or less, even though the same was 1 PPB previously. Sterner requests for high quality become manifest in other aspects: although previously contamination by fine particles with a diameter of 0.5 .mu.m or less was rejected, currently contamination by particles with a diameter of 0.2 .mu.m or less, and at a density of 100 particles/ml was rejected. This tendency requiring sterner standards for high quality is further emphasized nowadays: contamination by fine particles with a diameter of 0.1 .mu.m is regarded as problematic, and the tolerable limit is set at 100 particles with a diameter of 0.1 .mu.m or more for a unit space of 1 ml. In compliance with such stern requirement, has been eagerly wanted the advent of a polyethylene resin suitably used for the container for high purity chemicals, which is sufficiently clean to satisfy the current high standards regarding the contamination resulting from metal impurities and fine particles.
As a response to meet such expectation, Japanese Examined Patent Publication No. 5-41502 proposes a method whereby a high density polyethylene (HDPE) with a melt index of 0.1-8 g/10 minutes and density of 0.94 g/cm.sup.3 or more is employed for the manufacture of a container of hydrofluoric acid, but it does not give any description regarding the possibility of contamination by additives which have been added to the resin. Similarly, Japanese Examined Patent Publication No. 6-51399 discloses a container with a laminated structure suitable for the storage of sulfuric acid or the like whose outer layer is made of polyethylene containing a light shielding agent, and the innermost layer is made of a mixture of a metal salt of fatty acid and a hindered phenol anti-oxidant blended at a specified ratio to give a density of 0.958 g/cm.sup.3 or more, number average molecular weight of 5,000-12,000 and Mw/Mn of 15 or more. But the size of fine particles measured is 0.5 .mu.m or more, which means low level. Further, Japanese Unexamined Patent Publication Nos. 7-62161 and 7-257540 propose a method for the manufacture of a polyethylene container wherein the extraction amount of resin via a hydrocarbon solvent and the content of low molecular weight components are suppressed, and the addition of an anti-oxidant, neutralizing agent and light-fast agent is limited. The container, however, has a number of defects open for improvement: ash from a catalyst remaining in the polyethylene resin may give adverse effects; appropriate measures may be required for the prevention of metal impurities from dissolving into a chemical stored within; and the allowable level regarding the size of fine particles is set at 0.2 .mu.m or more, or a rather slack standard.
In addition to above, because it is often cumbersome to put a high purity chemical into small containers one after another, a demand is acute for the development of a method by which is obtained a polyethylene resin resistive to impact and suitable for the manufacture of big containers, because such a material would enable a great reduction in transportation cost.
Japanese Examined Patent Publication No. 4-20773 proposes a big container suitable for the storage of high purity chemicals which consists of an outer cylindrical portion made of FRP or the like and serving as a support, and an inner cylindrical portion fitting to the interior of the former and made of polyethylene. The polyethylene resin has a melt index of 0.01-0.04 g/10 minutes, and a density of 0.953-0.957 g/cm.sup.3, and does not substantially contain any additive possessed of a metal component. But, it discharges fine particles with a diameter of 0.5 .mu.m or more at a density of 10-20/ml. True, this container does not substantially contain any additive possessed of a metal component, and only includes an anti-oxidant consisting of a hindered phenol compound, but this alone would not be enough to suppress the number of fine particles with a diameter of 0.2 .mu.m or less down to 100/ml or less. Furthermore, the proposal by Japanese Examined Patent Publication No. 4-20773 requires the addition of an outer cylindrical portion for the mechanical support, which will complicate the involved production processes. Thus, it is necessary to develop a method by which a big container suitable for the storage of high purity chemicals is obtained without resorting to any mechanical support.